Current demands for high density and performance associated with ultra large scale integration require design rules of about 0.18 microns and under, increased transistor and circuit speeds and improved reliability. As device scaling plunges into the deep sub-micron ranges, it becomes increasingly difficult to maintain performance and reliability.
In the manufacture of semiconductor devices, photolithography is conventionally employed to transform complex circuit diagrams into patterns which are defined on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulator, conductor and semiconductor materials. Scaling devices to smaller geometries increases the density of bits/chip and also increases circuit speed. The minimum feature size, i.e., the minimum line-width or line-to-line separation that can be printed on the surface, controls the number of circuits that can be placed on the chip and directly impacts circuit speed. Accordingly, the evolution of integrated circuits is closely related to and limited by photolithographic capabilities.
An optical photolithographic tool includes an ultraviolet (UV) light source, a photomask and an optical system. A wafer is covered with a photosensitive layer, called resist, because of its ability to resist etchants. The photomask is flooded with UV light and the mask pattern is imaged onto the resist by the optical system. Photoresists are organic compounds whose solubility in a developer changes as a result of exposure to light or x-rays. The exposed regions become either more soluble or less soluble in a developer solvent.
There are, however, significant problems attendant upon the use of conventional photolithography to form patterns for subsequent processing. When a thin resist layer is coated on a reflective surface and exposed to monochromatic radiation, standing waves are produced in the resist. The reflected wave interferes with the incoming radiation wave and causes the light intensity to vary periodically in a direction normal to the resist. Standing waves cause variations in the development rate along the edge of the resist and degrade the image resolution.
For example, when a photoresist is coated on a highly reflective surface, such as silicon nitride which has an index of refraction of about 2.00, and exposed to monochromatic radiation, undesirable standing waves are produced as a result of interference between the reflected wave and the incoming radiation wave. In particular, standing waves are caused when the light waves propagate through a photoresist layer down to the silicon nitride layer, where they are reflected back up through the photoresist, and through the silicon nitride to the substrate, when they are again reflected to the photoresist.
These standing waves cause the light intensity in the resist film to vary periodically as a function of resist thickness, thereby creating variations in the development rate along the edges of the resist and leading to uncontrolled linewidth variations. These refections make it difficult to control critical dimensions (CDs) such as linewidth and spacing of the photoresist and have a corresponding negative impact on the CD control in superimposed layers of insulator, conductor and semiconductor materials.
Highly reflective transparent substrates accentuate the standing wave effects, and thus one approach to addressing the problems associated with the high reflectivity of the silicon nitride layer has been to attempt to suppress such effects through the use of dyes and anti-reflective coatings below the photoresist layer. For example, an anti-reflective coating (ARC), such as a polymer film, has been deposited directly on the silicon nitride layer. The ARC serves to eliminate reflection of most of the radiation that penetrates the photoresist thereby reducing the negative effects stemming from the underlying reflective materials during photoresist patterning. Unfortunately, use of an ARC adds significant drawbacks with respect to process complexity. To utilize an organic or inorganic ARC, the process of manufacturing the semiconductor chip must include a process step for depositing the ARC material, and also a step for prebaking the organic ARC or depositing a protective coating on the inorganic ARC before spinning the photoresist.
There exists a need for a cost effective, simplified processes enabling the formation of a self-aligned via which eliminate the need for a photomask and the negative effects stemming from the underlying reflective materials during photoresist patterning.
The present invention addresses and solves the problems attendant upon conventional multi-step, time-consuming and complicated processes for manufacturing semiconductor devices utilizing photolithography.